Exercise 3.69 - basic algorithm for generating triples with i <= j <= k... haven't done the testing code yet
enumerate all Pythagorean triples
Exercise 3.70 - merging streams of pairs according to some weight function
Exercise 3.71 - Ramanujan numbers!
Exercise 3.72: numbers that can be written as the sum of two squares in three different ways. the annoying part was expressing the answer as a stream...